Colour projection systems are known in the prior art. Consumer products, such as rear projection televisions, rear projection monitors and front projection systems are proposed in which liquid crystal on silicon (LCOS) micro displays are used. Several optical architectures exist for use with three LCOS panels, one each for red, green and blue. Achieving high contrast in a compact and cost-effective system remains problematic.
One existing architecture, referred to as the Color Quad™ architecture, proposed by Color Link uses four polarising beam splitter cubes in a square arrangement together with selectively polarising retardance plates such as the Color Select™ plates produced by the same company, to perform the colour splitting and polarisation functions necessary in an LCOS projection system. The system performs with high contrast, but the polarising beam splitter cubes are relatively expensive and require skew angle compensating retarders next to each LCOS panel in order to achieve high contrast. Also, the polarising beam splitter cubes are subject to strain birefringence, due to mounting or temperature induced stresses, further decreasing the contrast and the contrast uniformity, A broadband wire grid polariser for the visible spectrum is described in U.S. Pat. No. 6,208,463, assigned to Moxtek, and incorporated by reference herein. The company Moxtek produces wire grid polarisers which may be used as beam splitters. A paper, “An Improved Polarising Beam Splitter LCOS Projection Display Based on Wire-Grid Polarisers”, Arnold and Gardner, SIDOI Digest, 2001, describes the use of wire grid polarisers for a projection lens design architecture. In the paper, it is proposed to use a wire grid polariser and describes astigmatism produced by a tilted plate in a non-collimated beam as a problem in such as system.
It is an object of the invention to provide improvements in relation to these known devices.